The present invention relates to variable focal length lens systems, and, in particular, to variable focal length lens system suitable for use with a video camera or a digital still camera employing solid-state imaging device and the like, and relates to imaging apparatus equipped with the lens system.
Variable focal length lens systems with three-group configuration having negative-positive-positive refractive power arrangement suitable for solid-state imaging device have been disclosed in the Japanese Patent Laid-Open Publication No. 64-40913 and in the Japanese Patent Laid-Open Publication No. 6-94996, etc.
In each embodiment disclosed in the Japanese Patent Laid-Open Publication No. 64-40913, since the third lens group is configured with three or four lens elements, there are problems that the construction of the lens barrel for the third lens group becomes complicated and that it takes time for assembling and adjusting. Further, in each embodiment disclosed in the Japanese Patent Laid-Open Publication No. 6-94996, since the first lens group is configured with three lens elements, there are problems that the construction of the lens barrel for the first lens group becomes complicated and that it takes time for assembling and adjusting.
The present invention is made in view of the aforementioned problems and has an object to provide a variable focal length lens system with low cost, suitable for use with a video camera or a digital still camera employing solid-state imaging device, capable of assembling and adjusting with ease, and to provide an imaging apparatus equipped with the lens system.
In order to solve the problems, a variable focal length lens system according to the present invention has; in order from an object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power, wherein the first lens group consists of a cemented negative lens component, the second lens group comprises two positive lens elements and at least one negative lens element, and the third lens group consists of a single lens component, and wherein a space between the first and second lens groups is decreased and a space between the second and third lens groups is increased when a state of the lens group positions is changed from a wide-angle end state (a state providing the shortest focal length of the lens system) to a telephoto end state (a state providing the longest focal length of the lens system).
By providing a first lens group consisting of a single cemented negative lens component and a third lens group consisting of a single lens component, the assembling and adjusting of the first and third lens group become quite easy, and it is also effective for cost saving. Further, by providing a second lens group comprising two positive lens elements and at least one negative lens element, spherical aberration and coma can be well corrected.
It is further preferable that the cemented negative lens component is a cemented lens consisting of, in order from the object side, a positive lens element, and a negative lens element having double concave surfaces.
It is more preferable to satisfy conditional equation (1) shown below;
xcexd1Nxe2x88x92xcexd1P greater than 20xe2x80x83xe2x80x83(1)
where xcexd1N denotes Abbe number of the negative lens element in the first lens group and xcexd1P denotes Abbe number of the positive lens element in the first lens group.
Conditional equation (1) defines a preferable state where on-axis and lateral chromatic aberration is well corrected within whole variable focal length area. When the valuexcexd1Nxe2x88x92xcexd1P falls below the lower limit of conditional equation (1), it is not preferable that on-axis or lateral chromatic aberration becomes worse.
Further, in the present invention, it is desirable to satisfy conditional equation (2) shown below;
xe2x88x922.5 less than (R3+R1)/(R3xe2x88x92R1) less than xe2x88x920.5xe2x80x83xe2x80x83(2)
where R1 denotes a radius of curvature on the object side surface of the cemented negative lens component, and R3 denotes a radius of curvature on the image side surface of the cemented negative lens component.
Conditional equation (2) defines a preferable shape of the cemented negative lens component composing. the first lens group. When the value (R3+R1)/(R3xe2x88x92R1) exceeds the upper limit of conditional equation (2), the object side surface of the cemented negative lens component becomes strong concave surface, and it becomes difficult to correct astigmatism and distortion well. On the contrary, when the value falls below the lower limit of conditional equation (2), spherical aberration produced on the cemented surface becomes excessive, and correction of spherical aberration in the whole lens system becomes difficult.
Further, in the present invention, it is desirable that the third lens group consists of only a single positive lens element having double convex surfaces. It also helps for performing cost saving.
Furthermore, in the present invention, it is desirable that the third lens group consists of a single cemented positive lens component consisting of a positive lens element having double convex surfaces and a negative lens element having meniscus shape. With this configuration, on-axis and lateral chromatic aberrations can be corrected more preferably.
Further, in the present invention, it is desirable that the second lens group consists of, in order from the object side, two positive lens elements and one negative lens element. With this configuration, spherical aberration and coma can be corrected effectively.
Furthermore, in the present invention, it is desirable that the second lens group consists of, in order from the object side, a positive lens element and a cemented negative lens component wherein the cemented negative lens component consists of, in order from the object side, a positive lens element having double convex surfaces and a negative lens element having double concave surfaces. With this configuration, the construction of the second lens group can be simplified, and both spherical aberration and come can be corrected simultaneously.
Furthermore, in the present invention, it is possible for the second lens group to configure, in order from the object side, a positive lens element having double convex surfaces, a negative lens element having double concave surfaces, and a positive lens element having a concave surface facing to the object side.
Furthermore, in the present invention, it is desirable for the second lens group to consist of, in order from the object side, two positive lens elements, a negative lens element, and a meniscus lens element having a concave surface facing to the object side. With this configuration, spherical aberration and coma can be corrected effectively. Further, by suitably choosing the thickness of the meniscus lens element, the optimum exit pupil position for a CCD used in an imaging apparatus equipped with a variable focal length lens system according to the present invention can be realized.
Further, in the present invention, it is desirable for the second lens group to consist of, in order from the object side, a positive lens element, a cemented negative lens component and a meniscus lens element having a concave surface facing to the object side, wherein the cemented negative lens component consists of, in order from the object side, a positive lens element having double convex surfaces and a negative lens element having double concave surfaces. With this configuration, spherical aberration and coma can be corrected well, and it is possible to mitigate both spacing and decentering tolerance in the second lens group simultaneously.
Further, in the present invention, it is desirable to satisfy following conditional equations (3) and (4);
2 less than |f1/fw| less than 3.2(where f1 less than 0)xe2x80x83xe2x80x83(3)
0.8 less than f3/f2 less than 1.6xe2x80x83xe2x80x83(4)
where fw denotes a focal length of the variable focal length lens system in the wide-angle end state, f2 denotes a focal length of the second lens group, and f3 denotes a focal length of the third lens group respectively.
Conditional equation (3) defines an appropriate range of the focal length of the first lens group. When the ratio |f1/fw| falls below the lower limit of conditional equation (3), it is not desirable that spherical aberration produced in the first lens group becomes large, and that variation of spherical aberration in accordance with zooming becomes large. On the contrary, when the ratio exceeds the upper limit of conditional equation (3), it is not desirable that the lens system becomes large, and securing sufficient zooming ratio becomes difficult.
Conditional equation (4) defines an appropriate range of a ratio of the focal length of the second lens group to that of the third lens group. When the ratio falls below the lower limit of conditional equation (4), it is undesirable that spherical aberration and coma produced in the second lens group becomes excessively large. On the contrary, when the ratio exceeds the upper limit of conditional equation (4), the refractive power of the second lens group becomes large, and, as a result, it becomes difficult to correct aberration produced in the second lens group, or the configuration of the second lens group becomes complicated. It is undesirable.
Further, in the present invention, the first lens group moves, the second lens group moves to the object side, and the third lens group is preferably fixed when a state of the lens group position is changed from the wide-angle end state to the telephoto end state. As a result, the zooming mechanism can be simplified.
Further, the present invention is not limited to aforementioned configuration, and the zooming mechanism can also be simplified in such a configuration that the first lens group is fixed, the second lens group moves to the object side, and the third lens group moves when a state of the lens group position is changed from the wide-angle end state to the telephoto end state.
Further, it is desirable that the first lens group is moved to the object side when the focusing operation is performed from a far object to a near object. As a result, degradation of optical quality accompanied with focusing operation can be minimized.
Further, the present invention provides an imaging apparatus having a variable focal length lens system and an imaging device configured such that an image formed by the variable focal length lens system is detected by the imaging device. The variable focal length lens system is arranged, in order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power, wherein the first lens group consists of a cemented negative lens component, the second lens group comprises two positive lens elements and at least one negative lens element, and the third lens group consists of a single lens component, and wherein, in order to maintain imaging state, the first and third lens groups are fixed, the second lens group is moved to the object side, and the imaging device is moved when a state of the lens group positions is changed from the wide-angle end state to the telephoto end state. In this configuration, the zooming mechanism can be simplified.
Further, in the present invention, it is desirable that the imaging device is moved in the opposite direction to the object side when the focusing operation is performed from a far object to a near object. As a result, the moving mechanism of the imaging device can be used for both zooming and focusing.